There are a plurality of applications on large operating systems, such as mainframe and server operating systems, that place multiple datasets or objects on a tape volume. Examples include, but are not limited to, Data Facility Storage Management System (DFSMS) Hierarchical Storage Manager (HSM), DFSMS Object Access Method (OAM) and Tivoli Storage Manager (TSM). HSM and TSM may be used to migrate Direct Access Storage Device (DASD) datasets from one storage device to another, such as from a hard disk drive (HDD) to a tape drive, or to make a copy of a dataset, possibly to create a backup dataset. OAM places object data on a tape volume that may be a backup of data or original data. Typically, these applications access a database in which they keep track of the dataset/object name, the tape volume it was written to, the location on the tape volume of the dataset and/or object, and how many tape records make up the dataset/object.
When one of the migrated or backup datasets is requested by a user, these applications request a mount of the tape volume, and once the mount has completed, the applications instruct the tape drive to position to the location where the records associated with the dataset/object reside, and then read the requested records. This is typically referred to as a recall operation. If there are no other datasets on the tape volume to be recalled, the volume is demounted. The size of the dataset/object being recalled is often less than 250 KB in DFSMS HSM datasets, but may be any size in any system. There are also applications on open system platforms, such as TSM, that work in this way.
In physical tape drives, one of the key functions that is typical of an enterprise class tape drive is the ability to do a high speed locate operation to the beginning of the data to be read. This allows the tape drive to position to the requested data much faster than by just using conventional forward space block and read commands.
For a virtual tape storage (VTS) system that internally employs hierarchical storage (such as direct access storage device (DASD) cache and sequential access media), there are several reasons that a significant amount of inefficiency occurs when handling the above described types of application workloads. One of the biggest problems encountered when putting applications with this type of data on a VTS system is the time that occurs while waiting for a recall operation to retrieve the requested data. This is because the majority of the data written to a VTS system resides on sequential access media, such as tape cartridges, not in the DASD cache, which might be one or more a hard disk drives (HDDs). When data is requested that is only on sequential access media, it is recalled (copied) back into the DASD cache such that it is accessible to a host system. Currently, if the recall times are not acceptable to the user of the VTS system, native tape drives are added to the overall solution to replace non-native ones, and this can significantly increase the cost of the system to the customer. Therefore, it would be beneficial to have a system and/or method which could reduce the inefficiencies in accessing data on VTS systems which employ hierarchical storage.